This invention relates generally to audio amplifiers, and more specifically to means for controlling the power supply for the audio amplifier.
Solid state circuit components have brought incredible reduction in the size, weight and cost of audio amplifier circuitry and have also achieved increased fidelity in sound reproduction as compared with vacuum tube technology of a prior generation. In an attempt to exploit to the limit the potential of solid state circuitry, audio engineers have striven to provide the user with increased power ratings while simultaneously achieving decreased distortion levels. Their efforts have met with resounding success, but have produced some undesirable side effects primarily in the areas of increased weight, cost and power consumption. For example, a commercially available state-of-the-art 400 watt amplifier typically weights anywhere from 16 kg to over 38 kg depending upon the particular design and choice of materials. Such amplifiers normally employ costly components necessitated by the peak loads which they must carry, and generate significant amounts of heat which must be dissipated to avoid component damage.
With regard to the transformer weight problem an obvious approach would be to reduce the number of windings and/or the gauge of the wire making up the transformer coils. However, reduction in the number of windings also reduces the inductance in the primary coil, thereby increasing idling currents through the coil and contributing to both heat generation and increased power consumption. The conventional method for achieving low idling currents in the primary has been to use a large number of windings. This approach also requires a large number of windings in the secondary to keep the voltage in the secondary at the proper level. The other obvious alternative for weight reduction, i.e., reduction in the wire gauge, is not an acceptable solution since the internal resistance of each coil would be increased, leading to excessive heat generation and power loss upon high power demands beings placed on the transformer. Conventional wisdom has thus taught the necessity of increasing the size and weight of the transformer whenever a transformer powered amplifier is redesigned for increased power rating.
An alternative approach for reducing the overall weight, size and cost of audio amplifiers has been to reduce the total input power requirement without decreasing output power capability. Such increases in amplifier efficiency permit the use of less costly, lower weight power supplied, and can be achieved by reducing the power dissipation which normally attends the conventional use of output transistors in the output stage of the amplifier. When such power dissipation decreases are achieved, additional weight and cost savings are realized beyond those realized in the power supply since the weight, size and cost of the heat sinks normally required by the output transistors in the amplifier may also be reduced.
U.S. Pat. No. 3,426,290 to Jensen is representative of one known approach for increasing amplifier efficiency by keeping the voltage supplied to the output transistor of the amplifier very close to the output voltage level, thereby permitting operation of the output transistor in a condition which is at all times only slightly out of saturation. When operated in this condition, the actual voltage drop across the output transistor will be maintained quite low and the power dissipated by the transistor (equal to voltage across the transistor X current through the transistor) will be correspondingly reduced. A rather complex regulator is employed in the Jensen circuit to maintain the desired voltage supply to the output transistor wherein energy is stored in an inductive capacitive circuit by means of a switching transistor operated at high speed in response to a control signal derived from the audio input signal. By operating the switching transistor in full "on" or full "off" condition to maintain the desired voltage supply to the amplifier output transistor, energy consumption by the combined regulator and output transistor is reduced over that which would be consumed by an output transistor operated with a conventional fixed supply voltage. While producing a decided advantage in amplifier efficiency, the Jensen circuit is only truly effective if the switching transistor is operated at high frequencies, which can in turn cause transient interference distortion in the amplifier output signal. U.S. Pat. No. 4,054,843 to Hanada discloses a similar circuit to that disclosed in Jensen.
An alternative approach to achieving improved amplifier efficiency is disclosed in the patent to Dryden, U.S. Pat. No. 3,319,175, which discloses a stepped voltage supply operated in response to the voltage level of the amplifier output whereby the minimum voltage from the available power supply voltages sufficient to achieve the desired amplification is applied across the power amplifying element.
Still another approach disclosed in the prior art is illustrated in U.S. Pat. No. 3,622,899 to Elsenberg. In this patent a lower power dissipation amplifier circuit is disclosed including plural transistors coupled in series to a load terminal wherein the transistors are energized by respective voltage sources having different magnitudes and wherein the transistors are biased to operate as amplifiers in sequence in response to an input signal of increasing magnitude.
The patent to Schade, Jr., U.S. Pat. No. 3,887,898, discloses a transistor series amplifier wherein plural series connected transistors in the output stage are biased to share the total voltage drop in the output stage to permit use of lower cost components.
Still other techniques for reducing the cost of amplifier power supplies have been disclosed in the prior art. For example, in the U.S. patent to Munch, Jr., U.S. Pat. No. 3,542,952, a technique is disclosed wherein a single power supply may serve two Class B amplifier circuits designed to amplify the same audio signal by phase inverting the input to one amplifier to cause the amplifiers to draw peak current from the power supply in alternation.
None of the prior art systems discussed above addresses directly the problem of reducing power supply weight and costs by modifying the supply itself in a manner to employ less costly lighter, weight components while maintaining the power supply capabilities required by the amplifier circuit.
The patent to Chun, U.S. Pat. No. 3,466,527, discloses a circuit for reducing the cost and size of a transformer based voltage supply circuit including a duty cycle controlled switch in the A.C. power supply circuit of the transformer primary. The switch functions to regulate output voltage from the secondary. However, this patent does not in any way suggest how such a circuit design could be employed in an audio amplifier circuit in a manner to obtain power supply weight and cost reductions.
With regard to regulating power to the primary of a transformer, U.S. Pat. No. 4,051,425, Smith, shows a switching device comprising a triac connected in series with the primary winding of a transformer. There is a commutation circuit consisting of a second triac and a capactor connected in series with one another across the first triac. Subsequent to the first triac being turned on to initiate a current pulse through the primary winding of the transformer, the second triac is fired to cause the charge on the capacitor to appear across the first triac and overcome the current then flowing through the first triac to turn it off. The capacitor thereafter continues to discharge and because of the inductance in the primary coil of the transformer and a second inductor, current continues to flow in the same direction through the second triac and cause the charge on the capacitor to become reversed so that it is able to stop the current through the first triac during the next half-cycle when the second triac is again fixed. Experimental work by the applicant herein indicates that when high current is passing through the first triac, the capacitor must be made quite large to overcome the current and shut the triac off.
A search of the prior art has disclosed the following additional U.S. patents, and also a number of Japanese patents. These are as follows: U.S. Pat. No. 3,470,444, Bixby; U.S. Pat. No. 3,723,849, Ludlott; U.S. Pat. No. 3,506,905, Thomas; U.S. Pat. No. 4,115,742, Yokoyama; Japanese Pat. No. 50-141743; Japanese Pat. No. 49-58451; Japanese No. 50-455550.
In view of the foregoing, it is an object of the present invention to provide for an audio amplifying apparatus or the like, an improved apparatus and method for controlling the current pulses delivered to the primary winding of the transformer which supplies power for the amplifier or the like.
More specifically, it is an object of the present invention to provide an effective circuit by which the current pulse to the primary can be both turned on and turned off effectively with relatively simple and inexpensive circuit components.
Other objects will become apparent from the following detailed description.